
class python_graph_exception( RuntimeError ):
	"""
	A base-class for the various kinds of errors that occur in the the python-graph class.
	"""
	
class unreachable( python_graph_exception ):
	"""
	Goal could not be reached from start.
	"""
	def __init__( self, start, goal ):
		msg = "Node %s could not be reached from node %s" % ( repr(goal), repr(start) )
		python_graph_exception.__init__( self, msg )
		self.start = start
		self.goal = goal